The present invention relates to a thermal load reducing system for a nuclear reactor vessel to be used for the reduction of thermal load near the liquid surface of a reactor vessel and for the reduction of thermal load near a temperature stratified layer in a reactor vessel.
A reactor vessel in a fast breeder is supported by a concrete wall, an upper end of which must be maintained at the temperature of 100xc2x0 C. or lower. Because it has a high-temperature coolant at 500xc2x0 C. or higher in a plenum above reactor core, there occurs a steep temperature gradient in a vertical direction from the coolant liquid surface to the upper support unit. In particular, during starting operation, both temperature and liquid level are increased at the same time, and the gradient becomes steeper. As a result, high thermal stress develops generally near the liquid surface of the reactor wall where the temperature gradient reaches the maximum value.
To cope with this problem, attempts have been made in the past to prevent the increase of the liquid level using a liquid level controller, to evenly cool down the reactor wall using a low temperature sodium circulation system, and to reduce the flexural stress by designing in a thin-wall structure.
As described above, the conventional procedure for reducing thermal load has its principal aims to prevent the increase of liquid level using a liquid level controller, to evenly cool down the reactor wall using a low temperature sodium circulating system, and to decrease flexural stress by designing in thin-wall structure. The liquid level controller and the low temperature sodium circulating system result in higher cost because of the increase of system components. For designing the system in thin-wall structure, there is a limitation because of the possibility of operation failure.
In this respect, according to the present invention, partition plates are provided in an annulus space, and by a simple procedure to heat up the lower half of the gas space and to cool down the upper half of the gas space, and the temperature gradient itself is reduced, which is the cause to generate the stress developing near the liquid surface in the reactor wall. This contributes to further improvement of safety.
The thermal load reducing system for a reactor vessel according to the present invention is characterized in that partition members are arranged above a liquid surface of a coolant in an annulus space between the reactor vessel and a guard vessel, a low-temperature gas is circulated via the annulus space above the partition members to cool down, and the gas is circulated through the annulus space from under the coolant liquid surface to the partition members, and temperature above the coolant liquid surface is raised by high-temperature gas heated under the coolant liquid surface.
Also, the present invention provides the thermal load reducing system as described above, wherein said low-temperature gas is circulated at a constant flow velocity during operation, and said high-temperature gas is circulated only during starting operation.